A piston ring such as incorporated in an engine has typically been manufactured from a blank or a workpiece that consists of a "piston ring aggregate" formed of a plurality of non-circular rings stacked. The workpiece is machined over its inner and outer peripheral surfaces to provide inner and outer contours with a predetermined geometrical requirement. The aggregate so machined is then cut-split or -sectioned at two pistons in a direction of its longitudinal axis. Then, the two opposed surfaces of cut of the ring may be brought and fitted together, reducing the diameter of the rings, to provide a piston ring of the machined outer periphery with a required roundness and geometry.
A machine tool or apparatus dedicated to machine the inner and outer peripheral surfaces of a workpiece in the form of an aggregate of stacked non-circular rings has been known as proposed, e.g., in Japanese Unexamined Patent Publication No. Sho 54-21691 and Japanese Examined Patent Publication No. Hei 6-75814.
Japanese Unexamined Patent Publication No. Sho 54-21691 describes a machining apparatus characterized in that it comprises a work spindle on which a piston ring aggregate may be mounted, a tool support means capable of being displaced in a radius direction of the piston ring by an amount as a function of the curvature of a non-circular curved surface of the piston ring, and at least two computer controlled electro-mechanical stepping drive means for displacing the tool support means in a radius direction of the piston ring, arranged one behind another in the direction of their operation. The apparatus is described as being capable of simultaneously machining with due precision the inner and outer peripheral surfaces of a workpiece with non-circular curved surfaces composed of free running curves, by virtue of a stepping drive means that operates under computer control while the workpiece (made up of the piston ring aggregate) on the work spindle is in rotation.
Japanese Examined Patent Publication No. Hei 6-75814 describes a numerically controlled lathe in which a tool for cutting a workpiece is mounted on a carriage that is movable by a linear motor towards and away from the workpiece. With the linear motor operated under a computer assisted NC, the apparatus is designed to machine a skirt periphery on a workpiece such as for a piston ring. In this machine, a plurality of sets of rotary members and a biasing device are provided in a guide section that supports a reciprocation of the carriage, for the purpose of receiving a reaction force generated on the cutting tool while the workpiece is being cut. These means are described to permit the carriage to be reciprocated without jolting or deflecting in any way whatever and thus to permit the workpiece to be cut with precision.
The machining apparatus described in Japanese Unexamined Patent Publication 54-21691 noted above is seen to enable both the inner and outer peripheral surfaces of a workpiece to be machined simultaneously. It requires, however, that the so machined workpiece (piston ring aggregate) on removal to another site be then cut-split with a separate cut-split machining apparatus that is separately installed.
It is thus required that the workpiece that has been machined over its inner and outer peripheral surfaces be removed from the inner and outer peripheral surfaces of a machining unit having acted thereon and transferred to a system to the cut-split machining and then be there again indexed or located as to its phase as well as its three dimensional position. Such removal, transfer and repetitive indexing requirements involve time-consuming operations which have made the conventional system low in productivity and inconvenient.
Furthermore, not only does the need for the two separate apparatuses in a system demand a large space for installation of the entire system, but also these removal, transfer and repetitive indexing requirements entail the problem of mis-centering the workpiece to be cut-split which inconveniently has most often hindered the traditional system to yield a product piston ring of due precision that is acceptable for utilization.
On the other hand, a machining apparatus of the type described in Japanese Examined Publication No. Hei 6-75814 noted above is seen to be a numerically controlled lathe functionally limited to machining the outer peripheral surface of a workpiece and unable to work both the inner and peripheral surface simultaneously. Thus, not only does it give rise to a system which necessitates further a separate apparatus for working the inner peripheral surface to be worse in productivity, but such a system which requires the inner and outer surfaces of a workpiece to be machined separately makes the workpiece susceptible of being unequally aligned and centered in the two different apparatuses and hence is inconvenient as presenting a further machining inaccuracy problem.
The workpiece in such a system, having been so machined over both its inner and outer peripheral surfaces separately, must here again be transferred to and worked in a separate cut-split machine. Thus, the system here too is encountered with the same difficulties as mentioned above in connection with the system of Japanese Unexamined Patent Publication No. Sho 54-21691 and must be deemed to be more inconvenient as involving an increased space for installation and an increased cost and expenditure for equipment.
It is accordingly an object of the present invention to resolve the foregoing problems met in the prior art and to provide a machining apparatus, combined or of a composite type, for use in preparing a piston ring or the like product, in which a workpiece can be both machined over its inner and outer peripheral surfaces and subsequently cut-split, on the same single machining apparatus while it remains unaltered in position, i.e., held at an identical position therein.